Potassium carbonate (K2CO3) was chosen as the salt of choice. At material level (crystal level), the energy density is 361 kWh/m3 and it has an output temperature of about 60°C when hydrated with 10 °C water vapour pressure. Furthermore, it has no health or environmental issues and the cost is about 1 €/kg.
Initially, different K2CO3-powders were characterized. In order to arrive at a structurally more stable material, composite granules were produced and characterised.
Figure 2: Granules of potassium carbonate (K2CO3) that were produced for the CREATE system.
The material proofed to be stable for at least 100 cycles, sufficient for application as seasonal storage material.
Figure 3: Schematic representation of the TCM production, ready for mass scale.
The chemical industries involved developed and tested a production method for the salt hydrate that can easily be scaled up, and then produced about 1,5 tonnes K2CO3. The production steps are shown in Figure 3.
Figure 4: CREATE system module manufacture (Left to right; top to bottom: Heat exchanger top view; side view; 1m3 TCM material packed in individual containers; TCM material; Drum for mixing different TCM grain sizes.
Regarding the production technology, a study was made into the cost of the different unit productions that are needed and with this, an estimate could be made of the final cost of the salt hydrate when mass produced.
Table 1: Breakdown of the costs for stabilized salt hydrate, when mass produced.
The CREATE system is composed of a number of standard components and some developed component. CREATE developments are the modules and evaporator/condenser of the heat battery.
Figure 5: Schematic of the CREATE system.
The heart of the storage is a vacuum tight vessel that contains the salt in a heat exchanger. The low pressure is needed to enable evaporation of water in the evaporator/condenser in winter to discharge the storage.
Figure 6: Prismatic storage design reduces space needs by more than 20%
The developed module has a prismatic shape, leading to a more than 20% better use of the footprint in the boiler room than with standard, cylindrical containments. The vacuum forces are being taken up by the heat exchanger.
The heat exchange between salt and heat exchanger has been developed and tested in three steps: first, with a 1 kg set up.
Figure 7: The filled 1 kg heat exchanger.
In a subsequent development step, a larger absorber module of about 200 litres was designed, built and tested in the laboratory. With the module, charging and discharging power, cycling behaviour and temperature distribution were experimentally investigated.
The third step was the absorber module for the CREATE system. This is a prismatic containment of 1.58 by 0.95 by 0.35 meters, that can carry about 400 litres of K2CO3, see Figure 8.
Figure 8: Absorber module heat exchanger (left photo) and containment (right photo) before and after assembly and filling with salt hydrate (middle photo)
This first prototype absorber module was used in an experimental set up (see Figure 9) to determine the dynamic behaviour during charging, discharging and in stationary mode.
Figure 9: The first prototype absorber module (to the right, packed in insulation material), connected to the evaporator/condenser in the middle and to a hot water tank to the left.
A number of basic heat exchanger geometries were tested: fin and tube, microchannel, falling film and corrugated tube. The latter proved to be the best option, combining simple design with the best powers. A design was made for three tacked stainless steel dishes, each containing 5 meters of corrugated stainless steel tube, see Figure 10.
Figure 10: Corrugated tube evaporator/condenser.
All the developed components were assembled in the CREATE system, see Figure 11.
Figure 11: Schematic of the main components of the CREATE heat battery. On the left, the condensed water vessel with the evaporator/condenser built in. On the right, the three absorber modules containing 400 liters salt hydrate each.
After the HIL tests, the system was decommissioned from the laboratory and prepared for transport. In July 2019, the system was transported from Austria to Poland (Figure 13)
Figure 13: The container with the CREATE system in from of the laboratory (left) and start of shipment from Austria to Poland (right)
In Warsaw, the CREATE system was connected to the demonstration house. Figure 14 gives the main data on this house.
Figure 14: The CREATE demonstration house in Warsaw, Poland. The table gives the main characteristics of the demo house.
Figure 15: Installation of the vertical soil heat exchanger (left), the buffers (middle) and part of the measurement system (right) in the boiler room of the demonstration house.
After installation and commissioning tests, a 6-months demonstration test program was run in automatic mode and the system performed satisfactorily. There were no malfunctions. After the demonstration period, the system was decommissioned and the house went back to the standard heating system.
Throughout the project, the performance of the storage technology was tested and measured on different levels. The measurement results are summarised inTable 2.
Table 2: Overview of the experimental results with the three different volumes CREATE heat storage modules.
The The first prototype of 200 litres is denoted as WP6. The final absorber modules (WP7), were tested both in constant temperature mode (35 °C heat delivery and 10 °C evaporator temperature) and in Hardware-in-the-loop, HIL) configuration, in which the circumstances were much more dynamic.
The energy density of 115 kWh/m3 for the largest module in the HIL experiment is very good.
